Subproject 2.2

Role of plankton derived exopolymeric substances for air-sea exchange processes in the Anthropocene

Indoor Mesocosm
Experiment Summer 2013

Combined effects of
temperature and CO2 on organic matter dynamics in the surface
microlayer and consequences for air-sea gas exchange

The
extent and abundance of gel particles in the surface microlayer (SML) are suggested
to influence gas exchange at the ocean surface as well as the formation and
emission of primary organic aerosols. Hence, gel particles are assumed to play
an important role in exchange processes between the surface ocean and the lower
atmosphere. Gel particles may be of polysaccharidic or proteinaceous composition
and are therefore referred to as transparent exopolymer particles (TEP) or coomassie
stainable particles (CSP), respectively. They find their origin in dissolved
organic matter (DOM) precursors released by phyto- and bacterioplankton. At the
same time, gels represent hotspots for intense microbial activity, which may be
an important process implied in the continuous recycling of organic matter in
the SML.

In
order to study the formation and degradation of gel particles as well as their
effect on air-sea gas exchange under future ocean conditions, an indoor
mesocosm experiment was carried out from August to September 2013. Twelve mesocosms
were filled with ~1400L of fjord water including the natural plankton community
up to mesozooplankton and adjusted to two temperatures (16.5 and 22.5°C) and
six pCO2 levels (500 to
2500 µatm CO2).

Samples
were taken twice per week to analyse DOM (sugars, amino acids, coloured DOM)
and gel particles dynamics as well as bacterial biomass production, both in the
SML and the underlying bulk water. The size and abundance of TEP and CSP were analysed
microscopically. Simultaneously, gas exchange rates over the sampled SML were
quantified using N2O as a tracer gas. N2O is considered
to behave similarly to CO2 in the atmosphere and therefore enables
estimations concerning future CO2 uptake in the ocean via air-sea
gas exchange. Gas chromatography (GC) measurements were carried out weekly
using SML samples from two selected mesocosms.

Effect of bubbling
on organic matter enrichment in the surface microlayer

In the ocean,
dissolved organic compounds and TEP may adsorb on rising bubbles formed during
the breaking of waves. Bubble bursting at the ocean’s surface is responsible
for major emissions of sea-spraydroplets
to the overlying atmosphere. Furthermore, ascending bubbles enrich the SML with
organic compounds scavenged in the water column. To simulate this natural
process of organic accumulation in the SML, an additional experiment was
performed in continuation of the four-week temperature-CO2-mesocosm
experiment. The mesocosms were bubbled for 12h with ambient air at bubble sizes
of 25 – 1000 µm. Before and after bubbling, samples were taken to analyse DOM
and gel particles dynamics as well as bacterial biomass production both in the SML
and the underlying bulk water. The results of this experiment may help to
understand physical and biological processes in the ocean that lead to the
formation of an organic-rich SML. Furthermore, they will give estimates how these
processes may change in the future due to ocean warming and acidification.